Fuzzy rodents of the Rocky Mountains are giving scientists a hint of how hormone-mimicking chemicals can mess with animals’ reproduction.

Two new studies involving yellow-bellied marmots reveal that just slight changes in the animals’ natural exposure to testosterone has profound effects on their lives, according to Daniel Blumstein, a behavioral biologist at the University of California, Los Angeles.

For instance, female marmots exposed to more testosterone in the womb were less likely to survive their first hibernation, more likely to disperse, and less likely to become pregnant and wean young, the studies found. These “masculinized females” also acted more male by being more gregarious with other marmots, initiating more play and grooming sessions.

Two young male marmots greet. Photograph courtesy Raquel Monclús.

All this simply because the females happened to grow next to their brothers, which can essentially “leak” testosterone as part of the natural defeminization process in the uterus, Blumstein said.

To identify these male-like females, the scientists captured marmots in Colorado and measured the animals’ anogenital distances—or the lengths between the anus and genitals. Females exposed to more testosterone had longer anogenital distances than non-exposed females, he said. Such effects are more pronounced in females than males, as males start out with estrogen but lose the hormone as they develop.

But why the concern? There’ve been a slew of recent studies, mostly in the laboratory, about the impacts of so-called endocrine disruptors. Those are chemicals—such as those found in some plastics—that mimic estrogen or other sex hormones in the body, which can cause reproductive mutations in some animals.

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